1. Field of the Invention
This invention relates to methods of manufacturing polymeric medical devices, in particular, stents.
2. Description of the State of the Art
The discussion that follows is intended solely as background information to assist in the understanding of the invention herein; nothing in this section is intended to be, nor is it to be construed as, prior art to this invention.
Until the mid-1980s, the accepted treatment for atherosclerosis, i.e., narrowing of the coronary artery(ies) was coronary by-pass surgery. While effective and having evolved to a relatively high degree of safety for such an invasive procedure, by-pass surgery still involves serious potential complications and in the best of cases, an extended recovery period.
With the advent of percutaneous transluminal coronary angioplasty (PTCA) in 1977, the scene changed dramatically. Using catheter techniques originally developed for heart exploration, inflatable balloons were employed to re-open occluded regions in arteries. The procedure was relatively non-invasive, took a very short time compared to by-pass surgery and the recovery time was minimal. However, PTCA brought with it another problem, elastic recoil of the stretched arterial wall which could undo much of what was accomplished and, in addition, failed to satisfactorily ameliorate another problem, restenosis, the re-clogging of the treated artery.
The next improvement, advanced in the mid-1980s was use of a stent to scaffold the vessel wall in place after PTCA. This, for all intents and purposes, put an end to recoil, but did not entirely resolve the issue of restenosis. That is, prior to the introduction of stents, restenosis occurred in from 30-50% of patients undergoing PTCA. Stenting reduced this to about 15-20%, much improved, but still more than desirable.
In 2003, drug-eluting stents or DESs were introduced. The drugs initially employed with the DES were cytostatic compounds, compounds that curtailed the proliferation of cells that resulted in restenosis. The occurrence of restenosis was thereby reduced to about 5-7%, a relatively acceptable figure. Today, the DES is the default industry standard for the treatment of atherosclerosis and is rapidly gaining favor for treatment of stenoses of blood vessels other than coronary arteries such as peripheral angioplasty of the superficial femoral artery.
The next generation of stents will be those designed to be biodegradable. Although bioerodable metals may be used, biodegradable polymers are often used for fabrication of such a stent. However, there are potential shortcomings in the use of polymers as a material for implantable medical devices, such as stents. The strength to weight ratio of polymers is usually smaller than that of metals. Also, certain polymers have low toughness, i.e. are brittle. Semicrystalline polymers in particular are useful as stent material. However, they must be processed in a manner that provides high strength and fracture toughness.